1. Field of the Invention
The present invention relates to an endless material for security elements having micro-optical moiré magnification arrangements, and a method for manufacturing such an endless material.
2. Description of Related Art
For protection, data carriers, such as value or identification documents, but also other valuable articles, such as branded articles, are often provided with security elements that permit the authenticity of the data carrier to be verified, and that simultaneously serve as protection against unauthorized reproduction. The security elements can be developed, for example, in the form of a security thread embedded in a banknote, a cover foil for a banknote having a hole, an applied security strip or a self-supporting transfer element that, after its manufacture, is applied to a value document.
Here, security elements having optically variable elements that, at different viewing angles, convey to the viewer a different image impression play a special role, since these cannot be reproduced even with top-quality color copiers. For this, the security elements can be furnished with security features in the form of diffraction-optically effective micro- or nanostructures, such as with conventional embossed holograms or other hologram-like diffraction patterns, as are described, for example, in publications EP 0 330 33 A1 and EP 0 064 067 A1.
It is also known to use lens systems as security features. For example, in publication EP 0 238 043 A2 is described a security thread composed of a transparent material on whose surface a grating composed of multiple parallel cylindrical lenses is embossed. Here, the thickness of the security thread is chosen such that it corresponds approximately to the focal length of the cylindrical lenses. On the opposing surface, a printed image is applied in perfect register, the printed image being designed taking into account the optical properties of the cylindrical lenses. Due to the focusing effect of the cylindrical lenses and the position of the printed image in the focal plane, depending on the viewing angle, different sub-areas of the printed image are visible. In this way, through appropriate design of the printed image, pieces of information can be introduced that are, however, visible only from certain viewing angles. Through the appropriate development of the printed image, also “moving” pictures can be created. However, when the document is turned about an axis that runs parallel to the cylindrical lenses, the motif moves only approximately continuously from one location on the security thread to another location.
Also so-called moiré magnification arrangements have been in use for some time as security features. The fundamental operating principle of such moiré magnification arrangements is described in the article “The moiré magnifier,” M. C. Hutley, R. Hunt, R. F. Stevens and P. Savander, Pure Appl. Opt. 3 (1994), pp. 133-142. In short, according to this article, moiré magnification refers to a phenomenon that occurs when a grid composed of identical image objects is viewed through a lens grid having approximately the same grid dimension. As with every pair of similar grids, a moiré pattern results, each of the moiré strips in this case appearing in the form of a magnified and/or rotated image of the repeated elements of the image grid.
In manufacturing such moiré magnification arrangements, normally an endless security element foil is first manufactured as roll material, wherein, when conventional manufacturing methods are used, breaking points always occur, especially gaps or a misalignment in the appearance of the security elements. These breaking points come from the fact that the pre-products for the embossing dies used in manufacturing are generally manufactured as flat plates that are fitted on an impression or embossing cylinder. The image patterns that adjoin on both sides normally do not match at the seams and lead to motif disturbances of the kind cited in the appearance of the finished security elements after printing or embossing.